Enhance Python regrtest test runner (test.libregrtest)

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In issue gh-109162, I refactored deeply libregrtest code to easy its maintenance. While refactoring libregrtest, I saw many things that I would like to enhance. So I create this meta ticket to track enhancements :-)

Linked PRs

• gh-109276: libregrtest: use separated file for JSON #109277
• gh-109276: libregrtest only checks saved_test_environment() once #109278
• gh-109276: libregrtest calls random.seed() before each test #109279
• gh-109276: libregrtest: limit number workers #109288
• gh-109276: libregrtest: add RunTests.work_dir #109290
• gh-109276: Complete test.pythoninfo #109312
• gh-109276: libregrtest: fix worker working dir #109313
• gh-109276: libregrtest: WASM use filename for JSON #109326
• gh-109276: test.pythoninfo gets more test.support data #109337
• gh-109276: libregrtest: WASM use filename for JSON #109340
• gh-109276: libregrtest: WASM use stdout for JSON #109355
• gh-109276: libregrtest: fix work dir on WASI #109356
• gh-109276: Enhance libregrtest results #109828
• gh-109276: regrtest re-runs "env changed" tests #109831
• gh-109276, gh-109508: Fix libregrtest stdout #109903
• gh-109276: regrtest: add WORKER_FAILED state #110148
• gh-109276: regrtest: shorter list of resources #110326

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Recent libregrtest enhancements:

• Worker processes no longer import test.libregrtest.main: the python -m test.libregrtest.worker command is used instead. Change: commit a939b65.
• Worker processes are now run in a different working directory: in the temporary directory created by the main test process. Change: commit a939b65.
• --failfast now fails faster (!): RegressionTestResult.addFailure() now calls self.stop is support.failfast is true. Change: commit 31c2945.